My vote goes to this too. Specifically, not only the capacity but also the philosophy: to build things that are "good enough", but are easy to build in vast quantities. Others have already mentioned the vast numbers of Shermans and T-34's that were churned out. Individually, those tanks were no match for the German Tiger, but the numbers produced say all. By comparison, the Germans lost themselves in ever more convoluted designs.
This is something of a myth. It does have a good inkling of truth, but the Sherman is arguably just as gold-plated a design as the German big cats, with its advanced optics, gyroscopically stabilized gun, integral radios in every tank, wet ammo storage in later models,
leather padded interiors, etc. It developed an unwarranted reputation of being technologically inferior to the German cats first and foremost from the fact that the U.S. decided to continue producing
medium tanks for the logistical benefits and then pitted them against heavies on the defensive, not at all helped by green crews against veterans in bocage country.
The U.S., however, having the most advanced and efficient mass production technology in the world, was able to make complex weapons,
and do so in quantity with sufficient quality control that they don't face severe reliability issues. This can be seen in a wide variety of fields, like the Garand rifle: the only semi-auto rifle to see general service, the Packard Merlin: cranking engines out of an assembly line with better fit than the handmade British originals, the laminar flow wings on American fighters, the B-29, or 100+ octane avgas being widely available rather than carefully horded for high performance interceptors.
Less charitably, that the U.S. was able to goldplate its weapons and still outproduce everybody else combined made up for some less than inspired designs. A T-34 or Panther built with American goldplating and quality control would be fearsome indeed. And had the U.S. not poured more resources than the Manhattan project into the B-29, it would have been just as much of a convoluted and overly complex super advanced technological boondoggle as the ones Germany is so renowned for producing.
